Freezing temperatures or frost during early fruit development may injure apple fruit.

Identification

Mild cold damage can cause a corky, scabby ring on and around the surface of young fruit, usually near the calyx end. The corky areas enlarge as the fruit grow. On some fruit damaged by cold there may be no external injury symptoms. But as the fruit mature they become distorted or flattened due to internal pockets of dead tissue. Cutting open cold-damage fruit can reveal sunken pockets of brown, injured flesh. Sometimes fruit damaged internally by cold will drop from trees 1 to 2 weeks after the injury.

Disorder Development

Frost and freezing produce the same damage but occur under different conditions. Freezing occurs when air temperatures are 32°F or colder. Frost occurs when air is warmer than 32°F but plant tissues drop to 32°F or below because plants radiate (lose) heat into the atmosphere, especially during cool, clear nights.

The symptoms of cold damage depend in part on how cold temperatures were and for how long and the stage of plant development. Freezing or frost occurring from petal fall to when fruit are about 1/3 inch in diameter may cause internal freezing of the fruit.

The ability of apple flowers and fruit to withstand freezing temperatures is related to the presence and density of certain species of bacteria commonly found on plant surfaces. These bacteria serve as nuclei around which ice crystals form. In apple the fire blight organism, Erwinia amylovora, is a common ice-nucleating bacterium that can increase a tree's susceptibility to frost injury.

Damage

Freezing and frost can injure apple buds, flowers, fruit and leaves.

Solutions

To reduce the likelihood of frost injury, keep any cover crop or other vegetation under trees mowed short so that the soil surface is more exposed and firm. Unless soil is soggy, irrigate trees several days before cold temperatures are expected. These practices increase the soil's ability to absorb heat from sunlight and radiate the warmth at night to help keep trees warmer.

When a freeze or frost is predicted, if trees are small enough, covering them in the evening with blankets, frost cloths, or tarps of other than plastic can help keep plants warmer. Leave covers open at their bottom so heat from soil can help warm the plants. Remove covers during the day.

If freezing weather is predicted, covering plants alone is of little help unless a heat source is provided. Placing incandescent lights designed for outdoor use in the canopy may generate enough heat to prevent the plants from freezing if the plants are also covered. Be sure not to create hazards of electrical shock or fire.

If trees are damaged by a frost or freeze, do not remove dead leaves or twigs until late spring or summer. Prune out dead plant parts only after new growth develops. Pruning too early may remove live branches and increase the risk of more frost damage. An exception is limbs or trunks that are hazards and may fall; these should be promptly removed.

References

Adapted Integrated Pest Management for Apples and Pears, University of California Statewide Integrated Pest Management Program (UC IPM).